1. Field of the Invention
This invention relates to improved dentifrice abrasives. More particularly, the present invention relates to novel, alkaline earth-treated, precipitated silica abrasives which are suitable for use in therapeutic toothpaste compositions containing both soluble fluoride salts as enamel solubility reducing agents and soluble phosphate salts as dental pellicle film penetration agents. The invention further relates to methods for preparing these improved precipitated silica abrasives and to toothpastes containing the improved abrasives including toothpaste embodiments which comprise both enamel solubility reducing agents (i.e. fluoride) and dental pellicle film penetration agents. Such toothpaste compositions exhibit both high fluoride compatibility and high cleaning performance.
2. The Prior Art
The function of an abrasive substance in formulations intended for use in the oral cavity is to remove various deposits, including pellicle film, from the surface of the teeth. Pellicle film is tightly adherent and often contains brown or yellow pigments and thus imparts an unsightly appearance to the teeth. An advantageous toothpaste abrasive material should maximize film removal without causing undue abrasion to the hard tooth tissue. Dental researchers are continually concerned with developing toothpaste abrasives which demonstrate satisfactory levels of cleaning and which are not unduly abrasive and damaging to the oral tissue.
In addition to abrasives, therapeutic toothpastes typically contain fluoride ion sources. The beneficial reduction in the incidence of dental caries resulting from the topical application to dental enamel surfaces of solutions containing fluoride ions is well known. Especially at solution pH's between about 4 and 8, fluoride ions are believed to interact with enamel to reduce the acid solubility of such enamel. Enamel so treated with fluoride is more resistant to the formation of denta caries. Accordingly, therapeutic toothpaste compositions are formulated to provide fluoride ion availability in brushing solutions formed in the oral cavity during use.
It has been postulated that the effectiveness of fluoride treatment in providing enamel antisolubility/anticariogenic benefits is dependent upon the amount of fluoride ion which is available for uptake by the enamel being treated. It is, of course, therefore desirable to formulate toothpaste compositions which provide maximum fluoride ion availability in brushing solutions formed therefrom. However, efforts to utilize such ionic fluoride anticariogenic agents in toothpastes suitable for home use have been unable to provide the theoretical maximum soluble fluoride because of the tendency for ionic fluoride to be inactivated and thereby rendered unavailable for enamel uptake. That is, the toothpastes lose, upon storage (at rates which increase with temperature), the capability of providing the theoretical maximum amount of soluble fluoride. For purposes of this invention, the "soluble fluoride" content of any given toothpaste composition refers to the ppm concentration of fluoride ion which is found in the supernatant sample centrifuged from 1:3 by weight slurry of the toothpaste in water (1:3=toothpaste:water).
Fluoride ion sources tend to interact with toothpaste impurities and with such toothpaste components as abrasives, buffers, etc. Such interaction diminishes the ability of the fluoride source to provide "soluble fluoride" upon use. The propensity of the toothpaste compositions herein to maintain their levels of soluble fluoride after storage is expressed hereinafter as "toothpaste fluoride compatibility". Thus, the toothpaste fluoride compatibility of a particular toothpaste composition is that percentage of the theoretical maximum amount of fluoride source that is actually measured as soluble fluoride after storage for a specified time and at a specified temperature (e.g. one week at 120.degree. F.). Similarly, the propensity of such a dentifrice component such as the abrasive to interact with the fluoride source to diminish the measured "soluble fluoride" level from the theoretical maximum amount of fluoride sources (particularly in the presence of pellicle film penetration agents described in detail below) is expressed as "abrasive fluoride compatibility". The test procedures used herein to determine "toothpaste fluoride compatibility" values and "abrasive fluoride compatibility" values are described more fully hereinafter.
One toothpaste component which can pose special difficulties in formulating fluoride toothpastes is a precipitated silica abrasive component. Precipitated silica abrasives are desirable for use in toothpastes since they have desirably low dentin abrasion values. Certain prior art precipitated silica abrasives are generally compatible with soluble fluoride sources but have insufficiently high abrasivity to provide effective cleaning performance. Certain other prior art precipitated silica abrasives provide acceptable cleaning performance but have low abrasive fluoride compatibility as measured by the method hereinafter. It is believed that no prior art precipitated silica abrasives give both high "abrasive fluoride compatibility" as well as acceptable cleaning performance, (as indicated by standard Radioactive Dentin Abrasion values). There is thus a clear need to formulate precipitated silica abrasives which exhibit high "abrasive fluoride compatibility" as well as acceptable cleaning performance. Accordingly, it is an object of the present invention to provide precipitated silica abrasives which exhibit high "abrasive fluoride compatibility" as well as acceptable cleaning performance.
Another dentifrice component which can be especially destructive of soluble fluoride content in certain toothpaste compositions is soluble phosphate. Soluble phosphate salts, upon toothpaste use, serve to enhance the ability of fluoride ions to penetrate dental pellicle film. For this reason, soluble phosphate salts are desirably included in fluoride toothpaste compositions. However, particularly in combination with silica dental abrasives, soluble phosphate pellicle penetration agents tend to promote loss of soluble fluoride in toothpastes containing these materials and, thus, the toothpastes exhibit low toothpaste fluoride compatibility values. There is thus a clear need to formulate precipitated silica abrasives which provided high toothpaste fluoride compatibility when utilized in fluoride toothpastes containing soluble phosphate salts as pellicle film penetration agents.
There is thus a further need to provide fluoride toothpastes which can contain precipitated silica abrasives in combination with soluble phosphate salts. Accordingly, it is an object of the present invention to provide fluoride toothpaste compositions which contain soluble phosphate salts and precipitated silica abrasives and which nonetheless retain relatively high levels of soluble fluoride even after periods of storage.
It has been surprisingly discovered that the above objectives can be realized by the present invention which provides a novel precipitated silica abrasive which has been treated with an alkaline earth material, particularly calcium. By utilizing the instant dental abrasives, fluoride toothpastes--particularly those embodiments containing soluble phosphate salts--can be realized which have high toothpaste fluoride compatibility and excellent cleaning performance.
It is of course well known that therapeutic toothpaste compositions contain calcium phosphate materials as abrasives but these calcium materials are present in large amounts as described above and illustrated for example in U.S. Pat. No. 3,624,199, issued Nov. 30, 1971, Norfleet et al, and 3,864,471, issued Feb. 4, 1975, Mills et al. Toothpaste compositions are also known in the art which contain small amounts of alkaline earth metal ions, such as calcium ions, and compositions of this type are illustrated by U.S. Pat. No. 3,991,177, issued Nov. 9, 1976, Vidra et al. This patent discloses toothpaste compositions which contain a stabilizer-activator for a dextranase enzyme agent with the stabilizer-activator being a salt such as calcium chloride present in an amount of 0.001 to 0.3 weight percent. This composition can also contain therapeutic fluoride and the abrasive agent is calcium carbonate.
Other prior art which discloses toothpaste compositions containing alkaline earth metal compounds or ions include U.S. Pat. Nos. 3,095,356, issued June 25, 1963, to Moss; 3,122,483, issued Feb. 25, 1964, to Rosenthal; 3,669,221, issued June 13, 1972 to Hase; 3,782,446, issued Jan. 1, 1974, to Walter; 3,842,168, issued Oct. 15, 1974, to Colodney; and 3,689,537, issued Sept. 5, 1972, to Kuder. However, none of these prior art patents disclose therapeutic toothpaste compositions which contain as the abrasive agent a low structure precipitated silicon dioxide which contains about 10 to 300 parts per million of alkaline earth metal ion as described herein.